In modern electronics, although there have been countless developments in the field of optical communication, determining parameters like attenuation, free space path loss, etc has still been a significant issue. These factors play a highly crucial role in effective communication to ensure smooth and efficient data flow from one end to another.

Experiments have shown that chaos has distinct advantages in communication, where efficient use of a system's designated bandwidth increases the device's channel information-bearing capacity. This allows for more signal modulation than conventional systems while maintaining channel linearity by filtering out unwanted signals. Chaotic communication can also be operated at higher power levels, with safer communication. In optical communication, semiconductor lasers play a critical role owing to their compact size, efficiency, high speed, and modulation rates. They can be an investigative tool for determining single-mode semiconductor lasers as transmitters and receivers in the optical communication process.

General Methods used

The general method focuses on synchronization between the transmitter and receiver laser by equalizing different parameters. It also shows how the message encoding-decoding system affects this stability and induces unnecessary errors.
Various general models are used to achieve this purpose, some of them are:

A. Laser Dynamics: The dynamics of a single-mode semiconductor laser that is free running and operating above threshold can be explained via two conjoined first-order rate equations for carrier density N and intracavity photon density S.

Here, the cavity decay rate is C, spontaneous carrier decay rate is C, confinement factor is, J is injection current density, stochastic noise FS, and optical gain coefficient g which is related to N and S and can be stated by equation 2 as:

Equation 2

Other than relaxation oscillation between carrier densities and photons, there are no other significant dynamics in this two-dimensional continuous dynamical system. In order to increase the complexity of the dynamics in the system, we have to add more degrees of freedom which can be done via external perturbation. New and more effective techniques have been developed to perturb a single-mode semiconductor laser into chaotic oscillation.

Noise is another parameter that has a significant impact on the synchronization of lasers as it creates an unpleasant perturbation to the system, which reduces the quality and increases the bit error rate.

B. Synchronization: Many synchronization techniques have been developed over the years, but we are unable to implement these due to differences in semiconductor lasers and nonlinear dynamical systems. In order to synchronize two semiconductor lasers, we can make the transmitter and receiver laser follow two identical and dynamical sets of equations which can be represented as

Equation 3

Here, x and y represent dynamic vector variables of the transmitter and receiver respectively, s is the signal through which a message can be encoded, and D(s) which is force dependent on s. In order to achieve synchronization, we have to make the two vector functions F and G similar, hence achieving a stable solution y = x.

C. Message Encoding and Decoding: When a message-encoding mechanism is applied to a system, it is important to have a similar relationship between F and G, If the mechanism breaks this relationship, there would be no steady state(y = x). In this scenario, there will be no true synchronization between receiver and transmitter, and this can cause additional errors which ultimately produce additive and multiplicative chaos modulation. Moreover, these errors can make the synchronous chaotic state of lasers more complex as their dependency on temporal variations in the message has increased.

Optical Injection Methods used

Injection techniques lock the frequency and phase synchronization with respect to the photon-photon interaction when light is injected into the laser through the semiconductor device. Multiple optical injection methods systems were used: -

A. Laser Dynamics: As shown in the schematic figure below, the dynamics of an optically injected single-mode semiconductor laser cannot be set as per the photon density (S) and the carrier density (N). This can therefore be computed using the following equation

Equation 4

From the above equation, η is the injection coupling rate, Ai is the amplitude of the injection field at an optical frequency. By using laser dynamics, the optically injected single-mode semiconductor laser can be determined by five parameters, namely ⋎c, ⋎s, ⋎p, and b. In order to test the intrinsic dynamic parameters, the four-wave-mixing technique is used for a semiconductor laser.

Fig 1: Optical injection of a semiconductor laser

InGaAsP single-mode DFB laser at 1.3-m wavelength was observed together with the synchronization data. DFB follows a period-doubling route to chaos when one of its operational parameters is varied while the other two are fixed. Because of the laser noise, the state seen in the numerical mapping is very difficult to observe experimentally.

B. Synchronization: Fig 2 shows the schematic of the chaotic optical communication system as per the synchronization in unidirectional coupling. If arranged in the synchronization process, a coupling strength ∝ can be defined and the following equation can be computed:

Equation 5

The most robust synchronization occurs when the coupling strength is chosen to be around 0.5. The largest average transverse Lyapunov exponent is found to be negative for a sufficiently large coupling strength. The parameters of the transmitter and receiver lasers are closely matched to achieve this.

Fig 2: Schematic experimental configuration of a chaotic optical communication system

Two closely matched single-mode InGaAsP DFB lasers operating at 1.3m wavelengths were used as the transmitter and reception lasers in a synchronization experiment. A third InGaAsP laser served as the master laser and provided optical injection.

C. Message Encoding and Decoding: In this system, the message is carried by the injection field. This is an ACM-encoding scheme because the dynamics of the transmitter laser depend on the message-carrying injection field and the encoded signal. Message encoding and decoding data of this chaotic optical-communication system with optical injection. The recovered message is shown in the third trace, which is recovered by subtracting the receiver laser output from the received signal and then passing the residue signal through a filter. Message decoding at the receiver end is performed by inverting the encoding process with a subtraction operation.  Because the transmitted signal is in the form of an optical field, this chaotic communication system is a phase-sensitive coherent system.

Optoelectronic Feedback System of the Circuit

In this system, a photonic device with a feedback mechanism is used to create oscillation with low-phase noise. During the experiment, multiple feedback techniques were used. Some of them are as follows:

A. Laser Dynamics: The dynamics of a semiconductor laser with delayed optoelectronic feedback is shown in Fig 3. In this configuration, a combination of photodetector and amplifier converts the laser's optical output into an electrical signal that is fed back to the laser by adding it to the injection current. The phase of the laser field is not part of the dynamics of this system. In this configuration, a combination of photodetector and amplifier is used to convert the laser's optical output into an electrical signal that is fed back to the laser by adding it to the injection current.

Eqn 6

The dynamics of a single-mode semiconductor laser with optoelectronic feedback are determined by the same five intrinsic parameters, as mentioned above. The parameter does not have a direct effect on the dynamics of this system but only has an indirect effect through the coupling between the amplitude and phase noise fluctuations. In our experimental and numerical studies, we have found that a band-pass filter has the effect of changing the pulsing frequency but not the general dynamics of the system.

 Fig 3: Semiconductor laser with delayed optoelectronic feedback

B. Synchronization: Fig 4 shows the configuration of a chaotic optical communication system based on synchronizing unidirectionally coupled semiconductor lasers with optoelectronic feedback. The receiver laser in this system can have an optoelectronic feedback loop as well. A configuration where the receiver has an open loop without feedback is the easiest to implement and the most stable with the smallest synchronization error.

Fig 4: Structure of a chaotic optical communication system based on the synchronization

Eqn 7

Synchronization of the system in the absence of an encoded message was first studied by not adding a message to the signals. When the receiver laser is synchronized to the transmitter, it tracks the transmitter laser through different dynamical states as the controlling variable are varied.

C. Message Encoding and Decoding: Message encoding has to be implemented within the optical path of the feedback loop when the transmitter and the receiver are coupled through an optical link. Two possible methods of message encoding are. electro-optic modulations with the message multiplied on the output of the transmitter laser, which is MCM, as discussed above, and optical. modulation with the optical message added onto the transmitter's ACM. This chaotic communication system is ideal for digital messages encoded in the form of short pulses.

Experimental data of message encoding and decoding using an optical communication system with optoelectronic feedback was demonstrated. The experiment is conducted with the encoding and decoding message of a stream of pulses at a repetition rate of 100 MHz. The time series of the message encoding and decoding showed that the different traces have the same meaning as the corresponding ones. The recovered message shows a very good quality of decoding compared with the encoding message.

Conclusion

Two systems, one with optical injection and the other with delayed optoelectronic feedback, were modeled and studied throughout the lifecycle of the experiment. Message encoding and decode schemes can be implemented in these systems while preserving the identity of the transmitter and receiver systems so that true synchronization is maintained. A single-mode semiconductor laser can be modeled as a nonautonomous nonlinear system of three dynamical variables.

Research results show the feasibility of chaotic optical communications based on two semiconductor laser systems. They open up many research opportunities on both theoretical and experimental sides. These experimental results are focused on implementing very high bit rate message encoding and decoding through muti-giga-hertz chaotic signals. Further research and development in this field can include the optimization of encoding schemes based on chaos modulation, signal processing algorithms for message decoding, and multi-channel and multiplexing systems.